When an optical member such as a polarizing plate is stuck to a liquid crystal display panel, a pressure-sensitive adhesive is generally used. For the pressure-sensitive adhesive for an optical member, there are required various properties, such as (1) heat resistance, that is, even if it is placed under the high temperature conditions, foaming, lifting, peeling, etc. do not occur at the adhesive part, (2) wet heat resistance, that is, even if it is placed under the high temperature and high humidity conditions, foaming, lifting, peeling, etc. do not occur at the adhesive part, (3) light leakage prevention property, that is, even if the optical member undergoes shrinkage under the high temperature conditions to thereby place the pressure-sensitive adhesive having been applied to the optical member under stress, unexpected light leakage is not brought about in the display, and (4) aging property, that is, the number of days between the day the curing agent is added to the pressure-sensitive adhesive composition and the day the curing reaction is completed to stabilize the properties of the pressure-sensitive adhesive is small.
Particularly in the production of polarizing plates used for liquid crystal displays or the like, the aging period of the pressure-sensitive adhesive has great influence on the lead time, and therefore, the aging property is one of important properties among the above properties. In general, the pressure-sensitive adhesive composition for a polarizing plate is mixed with a curing agent, and thereafter, the mixture is processed into a work in process in the form of a roll (three-layer structure of polarizing plate/pressure-sensitive adhesive layer/release film or release film/pressure-sensitive adhesive layer/release film, or the like). In order to complete the curing reaction to exhibit given properties required, this work in process is stored in an aging room preset at 20 to 50° C. (usually 40° C.) for about one week. Thereafter, the work in process is cut to a desired size, then the release film is peeled off, and the work in process is stuck to a liquid crystal cell. Because of such a working process, the longer the storage period in the aging room is, the more the works in process required to be stored are, and the cost of temperature control in the aging room or other running costs are also increased. Therefore, the need of pressure-sensitive adhesive compositions for polarizing plates, the aging period of which is short and which do not need to be stored in the aging room, has increased.
In these days, as a method to shorten the aging period, there is a method of using a metal chelate-based curing agent as a curing agent or a method of adding a curing accelerator to a monomer mixture before polymerization. A pressure-sensitive adhesive prepared by the use of the metal chelate-based curing agent, however, generally has poor heat resistance, and therefore, use of the metal chelate-based curing agent is not a preferred choice from the viewpoint of durability under the severe conditions.
In the case of adding a curing accelerator, the thickening action of the pressure-sensitive adhesive with time generally appears early and markedly strongly, and thereby, viscosity increase of the coating solution is brought about. Hence, application of the pressure-sensitive adhesive often becomes difficult. In the case where gelation proceeds in early stage and the viscosity of the coating solution of the pressure-sensitive adhesive becomes excessively high as above, smooth coating cannot be carried out unless a pressure-sensitive adhesive coating solution using a large amount of an organic solvent is used, and besides, there also occurs a problem that in order to form a pressure-sensitive adhesive layer, a large amount of an organic solvent must be removed.
Further, there is a case where an amine-based compound is used as such a curing accelerator, but for example, a pressure-sensitive adhesive using a polyhydroxyalkylamine-based curing accelerator is liable to undergo yellowing due to change with time, and such a curing accelerator is unsuitable for use in a pressure-sensitive adhesive for optical use. Furthermore, there is also a case where an amino group-containing monomer is copolymerized to allow the resulting acrylic polymer to have a curing acceleration action, but in the system using such an amino group-containing monomer, the curing reaction excessively proceeds, and as a result, reduction of tack of the resulting pressure-sensitive adhesive is sometimes brought about.
There is also a case where an organometallic compound-based curing accelerator is used, but for example, an organotin compound commonly used as the organometallic compound-based curing accelerator has high toxicity, and in particular, tributyltin contained in dibutyltin dilaurate is an anxious substance as an endocrine disruptor. Also from this viewpoint, use of the organometallic compound-based curing accelerator is not recommended.
On the other hand, according to the prior art literatures, a pressure-sensitive adhesive composition for a polarizing plate, which is capable of sufficiently undergoing curing due to curing reaction in the coating/drying step without needing aging by incorporating a specific amount of a carboxyl group-containing polymer in a mixture of a hydroxyl group-containing polymer, particularly a (meth)acrylic polymer, and an isocyanate-based curing agent, has been disclosed in a patent literature 1. In the patent literature 1, however, studies of light leakage that is an essential evaluation item for pressure-sensitive adhesives for polarizing plates have not been made at all, though durability and aging property of the pressure-sensitive adhesive have been taken into accounts.
In a patent literature 2, there has been disclosed a pressure-sensitive adhesive composition, which comes to have a long pot life, proper tack, small change of tack with time and good durability in the heat treatment by adding an aliphatic isocyanate-based compound and/or a polyfunctional isocyanurate-based compound, and a compound causing keto-enol tautomerism to an acrylic copolymer containing a monomer having a hydroxyl group and a monomer having a carboxyl group. In the patent literature 2, however, studies of durability and light leakage required for pressure-sensitive adhesives for polarizing plates have not been made because a protective film has been supposed as a use purpose of the resulting pressure-sensitive adhesive, though a curing agent having an isocynurate skeleton has been used.
In a patent literature 3, there has been disclosed a pressure-sensitive adhesive composition for an optical member, which can be improved in processability owing to shortening of aging time in the curing treatment by adding a silane compound having an amino group and an isocyanate-based curing agent to an acrylic polymer comprising a monomer having a hydroxyl group and which exhibits durability and reworkability. In this literature, studies of durability and reworkability of the composition as a pressure-sensitive adhesive for a polarizing plate have been also made, and in order to shorten the aging period, an amino group-containing substance has been used.
In the patent literature 3, however, studies of light leakage have not been made, and the pressure-sensitive adhesive composition is insufficient as a pressure-sensitive adhesive for a polarizing plate.
In a patent literature 4, there has been disclosed a pressure-sensitive adhesive composition for an optical member, which is obtained by adding a metal chelate-based curing agent and an isocyanate-based curing agent to an acrylic polymer obtained by copolymerizing a monomer mixture containing at least a monomer having an alkyl group of 1 to 16 carbon atoms and a monomer having a carboxyl group and optionally containing acrylamide. However, the resulting pressure-sensitive adhesive has poor heat resistance and is insufficient as a pressure-sensitive adhesive for a polarizing plate, though the metal chelate-based curing agent contributes to shortening of the aging period of a pressure-sensitive adhesive, as described above.